Greater Manchester police have arrested a 15-year-old boy in relation to the alleged sending of a racist tweet to Manchester United's Rio Ferdinand.

The tweet was sent during United's 3-2 derby victory over Manchester City on Sunday. A statement on the GMP website said: "Police investigating a racist tweet sent during Sunday's Manchester derby have made an arrest. An investigation was launched following a message directed at a Manchester United player.

"A 15-year-old boy was arrested on suspicion of a racially aggravated public order offence on the evening of Monday 10 December 2012. He has been bailed pending further inquiries."

In unrelated incidents Ferdinand was hit by a coin and approached by the 21-year-old Manchester City supporter Matthew Scott, who had encroached on the pitch as the defender celebrated Robin van Persie's late winner.

Police made nine arrests as tempers among rival fans boiled over and officers have appealed for anyone with information about hooligans throwing coins to get in touch.

Patrice Evra believes City lost the derby because of Roberto Mancini choosing to leave out Carlos Tevez for Mario Balotelli in his starting XI. "I was really happy when he [Tevez] didn't start the game," United's captain told the Manchester Evening News. "I told him before the game. It was a good sign for us."

Van Persie has praised the influence of Javier Hernández and Danny Welbeck, the youngest members of United's four-strong strikeforce, on him since joining the club.

The Dutchman's winner against City was his 14th goal for United since his £24m arrival in the summer. The 29-year-old and Wayne Rooney are Sir Alex Ferguson's first-choice forwards but Van Persie, asked if Hernández and Welbeck can learn from him, told MUTV: "It goes both ways – I can learn from them as well. For example, I love Chicharito's [Hernández's] enthusiasm. I love how he plays and trains and does his work off the pitch after training. A couple of weeks ago he wasn't playing as much but he was in the gym every day doing extra work and that sets a big example for everyone.

"Danny, too. He's not playing in his favourite position at the moment but we need him and he's working hard. So when you have four strikers being so professional, it sets the tone for the others. Everyone realises we are all in it together and we all need to do our bit."

Evra praised Rooney's recent resurgence. "At the beginning of the season a lot of people were criticising Wayne, which was normal because Robin joined us and is doing great, but you can't forget Wayne and he just proved it," he said. "He scored two goals in the last game against Reading and he's scored another two goals again against City. In the big games you need your big players and Wayne is one of them."

Rooney has seven goals in all competitions this season, Hernández nine and Welbeck one.